1. Field of the Invention
The present invention is directed generally to underground manhole assemblies and more particularly to a method and arrangement for preventing the assembly from moving upwards in response to a change in surrounding soil conditions.
2. Discussion of the Prior Art
As is generally known, manhole assemblies are often provided as a juncture for joining underground sewer pipes which extend in different directions therefrom. Typically, a manhole assembly will include a base which sits directly against the soil beneath it. In relatively dry or warm climates, the soil surrounding the manhole assembly and particularly the soil beneath its base will remain in an unfrozen and/or unexpanded state. However, in moist climates where freeze and thaw conditions prevail, moisture directly beneath and directly against the manhole assembly tends to freeze, causing the soil to expand in an upward direction against the base of the manhole assembly. This, in turn, can force the assembly towards ground level.
When the frozen soil thaws, the side pressures exerted against the assembly prevent the ladder from moving back to its original position and eventually there is further soil build-up beneath the base. Hence, at the next freeze period, this procedure is repeated. While each increment of upward movement of the assembly is small, after many freeze-thaw cycles, the entire manhole assembly can move upwards a substantial distance.
The foregoing creates two major problems. Firstly, if the manhole assembly is located beneath a street or sidewalk, which is generally the case, upward movement of the assembly can cause substantial damage to the street or sidewalk. This, of course, can create hazardous conditions. Secondly, since the manhole acts as an extension or juncture to sewer pipes, substantial movement of the manhole produces misalignment which, in turn, interferes with the otherwise smooth flow through the sewer pipes. This, in turn, can produce a pressure problem within the sewer pipes and also can create a static condition within the latter, causing sewage to settle in the manhole, thereby creating a maintenance problem.
The aforedescribed freeze-thaw conditions can also exist along the sides of the manhole assembly. In this regard, many manhole assemblies are constructed in sections which are stacked on top of one another. It has been discovered that ice along the periphery of a given section of the assembly, during the freeze cycle, grips the assembly section as the ice expands and raises the section upward. As this occurs, gravel and/or other debris may collect between the raised section and the section directly below it, preventing the raised section from returning to its normal position. After many freeze-thaw cycles, the overall upward movement of any given manhole section may be substantial, resulting in the same problems discussed above.